The present invention relates to a device for detecting a pneumatic pressure drop of each of tires provided for a four-wheel vehicle and, more particularly, to a tire deflation warning system for more reliably judging a tire pneumatic pressure drop in both cases when a driving wheel is being reduced pressure and a driven wheel is being reduced pressure.
Conventionally, there has been practically used a method that detects whether or not a pneumatic pressure of a tire is dropped by using rotational velocity of a wheel provided for a vehicle. A tire pressure abnormal alarming method which informs of a pneumatic pressure drop of a tire provided for a vehicle judges by relative comparison of four-wheel rotational velocities.
Since reduced pressure wheel becomes fast in rotational velocity, in a tire deflation warning system (DWS), a reduced pressure judging value (DEL value) uses an equation,DEL=(Vfl+Vrr−Vfr−Vrl)/Vmean×50where Vfr is rotational velocity of a front right tire (a dynamic load radius×rotational angular velocity, same as above hereinafter), Vfl is rotational velocity of a front left tire, Vrr is rotational velocity of a rear right tire, and Vrl is rotational velocity of a rear left tire. By using this, it can be seen that a certain wheel is being reduced pressure from that the DEL value (an absolute value thereof becomes large from zero when a certain position is being reduced pressure. In the DWS, an alarm is set to activate, assuming that it is a reduced pressure state when the DEL value exceeds a specified value (for example, 0.13). The specified value is called as a reduced pressure judging threshold value.
On the other hand, even in the same state as the reduced pressure state by 30%, when the case at 50 km/h is compared with the case at 100 km/h, a tire radius at 100 km/h becomes larger by centrifugal force and the difference in the rotational velocity between a reduced pressure wheel and a normal pressure wheel will become small, resulting in a phenomenon that a DEL value in the case at 100 km/h becomes small with respect to a DEL value in the case at 50 km/h and come close to a DEL value at a normal state. For example, when the front wheel is being reduced pressure by 30%, the DEL value at 50 km/h becomes DEL=0.15, whereas the DEL value at 100 km/h becomes DEL=0.1.
In the DWS, since the threshold value (for example, 0.13) at which alarms is set to constant with respect to the velocity, a variable coefficient (a weight coefficient being velocity function, refer to FIG. 8) with respect to the velocity to the DEL value is multiplied so as to correct a drop of the DEL value at high speed when there is a reduced pressure wheel and to compare with the reduced pressure judging threshold value which is constant with respect to the velocity, and thus judges whether or not it is a reduced pressure state.
As shown in FIG. 9 (A vehicle BUICK RENDEZVOUS CVL, whose front wheels are driving wheels and rear wheels are driven wheels), since the rotational velocity of the reduced pressure wheel becomes fast, the DEL value is not zero, but becomes large, for example, 0.15. When traveling at high speed, the tire radius of the reduced pressure tire becomes large by the influence of the centrifugal force, and consequently the difference in the rotational velocities between the reduced pressure tire and the normal pressure tire becomes small to come close to zero (refer to FIG. 9, the range from 90 km/h to 140 km/h).
This influence becomes larger in the case when the driving wheel, to which a driving torque is applied, is being reduced pressure than in the case of the driven wheel, and as a result, the DEL value is rapidly dropped (from 90 km/h to 140 km/h of FIG. 9). This decrease is referred to as “an acute drop of a remarkable DEL value produced at high speed of the driving wheel reduced pressure or a value of similar equation”.
Consequently, a tire pressure drop detecting method which corrects a reduced pressure judging value by a weight coefficient according to a velocity and corrects reduction in reduced pressure sensitivity at high speed, has been known. For example, as shown in FIG. 8, the reduced pressure judging value is weighed by the weight coefficient according to the velocity (multiply the DEL value by the weight coefficient) and corrects reduction in DEL value, thereby enabling judgment at high speed.
Other techniques have been known, for example, a method for detecting pneumatic pressure by selecting or weighing either one of judged results, according to a vehicle traveling state including a road surface state, i.e., a judged result of pneumatic pressure based on a resonance frequency or a judged result of pneumatic pressure according to comparison of a rotational velocity element of a plurality of tires (refer to Japanese Unexamined Patent Publication No. 297923/1994); a method for correcting a reduced pressure judging value by a weight coefficient according to a vehicle velocity in addition to judging a pneumatic pressure drop according to the range of front/rear accelerations of a vehicle (refer to Japanese Unexamined Patent Publication No. 203678/1997); a method for correcting a tire pressure drop judging value according to a vehicle velocity (refer to Japanese Patent No. 3135437 and Japanese Patent No. 347516); and a method for correcting a rotational angular velocity by a ratio between a previously determined driving wheel and driven wheel according to a tire rotational angular velocity (refer to Japanese Unexamined Patent Publication No. 137512/1995).